Big river, big study

Amazon outflow:
True-color image of the Amazon River outflow, which extends thousands of kilometers into the Atlantic Ocean.

Steinberg-led VIMS team to join Amazon River research project

by David Malmquist
| May 18, 2010

Professor Deborah Steinberg of the Virginia Institute of Marine Science,
College of William & Mary, will join an international team in late May and
June for the first part of a three-year study of how the Amazon River's huge
freshwater plume affects the biology and chemistry of the Atlantic Ocean.

The $1.2 million project is funded by the National Science Foundation.

The team's goal is to better understand how ocean plankton and river-borne
nutrients interact to affect the global carbon cycle and climate change. That
goal is spelled out in their project title and river-themed acronym: Amazon
iNfluence on the Atlantic: CarbOn export from Nitrogen fixation by DiAtom
Symbioses (ANACONDAS).

The ANACONDAS team-which also includes researchers from
the Bigelow Laboratory for Ocean Sciences, Georgia Tech, San Francisco State
University, the University of Georgia, the University of Maryland Center for
Environmental Sciences, the University of Southern California, and Brazil-will
sail from Barbados in late May aboard the 279-foot RV Knorr for a 5-week
expedition to the open waters of the western equatorial Atlantic, where the
Amazon pours up to 11,000,000 cubic feet of freshwater into the ocean every
second. By contrast, the Chesapeake Bay is the nation's largest estuary, with 15
trillion gallons of water. The Amazon River can discharge that much water in two
days.

"The size and power of the Amazon are incredible," says Steinberg. "Its total
flow exceeds the next 10 largest rivers combined, and it lowers the salinity of
the Atlantic more than 300 miles out to sea. Its plume can affect the ocean up
to 1,000 miles offshore.

"Our goal," she continues, "is to determine how marine plankton and bacteria
use nutrients in the plume, and how that can ultimately lead to transfer of
carbon dioxide from the atmosphere to the deep sea."

Carbon that reaches the deep sea can remain there for thousands of years and
contributes nothing to current global warming.

Food Webs and Carbon Dioxide

Steinberg says that marine scientists "traditionally viewed the open waters
of the tropical Atlantic as a nutrient-poor, ‘retention' food web, in which the
CO2 that plankton take up during photosynthesis is recycled within surface
waters because the plants are too small to sink very fast on their own, and the
grazers that eat them are smaller too." Carbon dioxide in these waters typically
reaches a balance with the level of CO2 in the atmosphere, which currently
stands at 389 parts per million (ppm).

But an earlier study-led by
members of the ANACONDAS team-revealed that levels of carbon dioxide in tropical
Atlantic waters influenced by the Amazon plume measure 239 ppm, far below the
atmospheric concentration. That led the researchers to suspect that a different
food web was operating in the plume area-namely the type of "export" food web
more typically found in nutrient-rich coastal waters.

"In export food
webs," says Steinberg, "larger plant plankton such as diatoms dominate. The
zooplankton that graze on them are also larger. When these larger and heavier
plants die, or zooplankton eat them and release fecal pellets, the carbon in
their tissues is much more likely to sink to deep waters."

This export of
carbon to depth-what marine scientists call the "biological pump"-allows the
surface waters to soak up more CO2 from the atmosphere, thereby reducing the
greenhouse effect.

Nitrogen-Fixing Bacteria

The catch, though, is that the Amazon's waters are relatively poor in the
forms of nitrogen that plankton need for growth. Without these nitrogen
compounds, resident plankton would be unable to tap the river's other more
abundant nutrients, particularly phosphorous, and the biological pump would
falter."In the plume," says Steinberg, "evidence suggests that the nitrogen
needed to prime the biological pump is supplied by nitrogen-fixing bacteria."
Like the bacteria that inhabit the roots of peas and other legumes on land,
these marine "diazotrophs" are able to extract nitrogen gas directly from the
environment.

"One of these diazotrophs-Richelia-lives in an amazing symbiosis," says
Steinberg. "It lives just below the glassy shell of the diatom, providing it
with nitrogen for growth and in turn receiving protection and buoyancy. From
prior work we know that DDAs [diatom-diazotroph assemblages] are responsible for
a significant amount of the CO2 drawdown in the Amazon plume."

During the upcoming expedition, the VIMS team-Steinberg, marine technician
Joe Cope, graduate student Brandon Conroy, and recent William & Mary
graduate Miram Gleiber-will deploy large nets at a series of sampling sites
inside and outside the plume in order to collect and identify zooplankton from
different depths. They will then use on-board and later laboratory experiments
at VIMS to better understand what and how much the different zooplankton species
are eating, and how much carbon they export through fecal pellets and other
mechanisms.

Their efforts will complement work by shipmates aboard the Knorr to further
study the resident phytoplankton and bacteria; record levels of nitrogen,
phosphorous, and other elements; map the plume's extent using satellite imagery;
and measure how much and how fast carbon sinks to depth beneath the plume.

"The ultimate goal of the project," says Steinberg, "is to quantify and model
the plume's influence on the global carbon cycle, and to understand how its
influence might change with a changing climate."